For the "observe the grenade" achievement, the video is helpful and I think those are the only "testers" on the level. Everyone else prior to the final battle is not. So it has to be done here.
When the testers spawn you are on the left side of the level. Generally 2 will spawn in the middle and 1 on the left side. If you run down the stairs towards the one on the left, he should move right and the 2 from the middle should come up the small set of stairs. Once at the top of the stairs they basically group up. At that point I just ran into them and fired 1 grenade.. I killed myself.. but I also killed them and got the achievement. Probly took an hour to get it right.. Originally I was shooting out their cover to group them.. but they'd always end up killing me. Hope it helps!
Completed RPG's: 51
Proudest: Star Ocean; Dark Souls; Neverwinter